Gas turbine engines may be provided with main and auxiliary fuel supplies for supplying fuel to the engine via an engine fuel line. Although not limited to gas turbine engines for aircraft, the following discussion will generally be directed to such engines. The main fuel supply is generally provided by a main fuel pump driven by the gas turbine engine. The auxiliary fuel supply may be provided by an auxiliary pump, such as a variable displacement pump, that may be used for supplying fuel to other devices after engine startup. The auxiliary pump is used during engine start up before the engine has reached operating speed, and is also used to provide a back up fuel supply in the event of a problem with the main fuel supply. In addition, under some operating conditions, such as during stationary take-off/vertical landing (STOVL) or during other critical maneuvers it is important to reduce the risk of fuel supply interruption. At these times, both the main fuel supply and auxiliary fuel supply may be simultaneously connected to the engine fuel line to better ensure a quick switch from one fuel supply to the other if one of the fuel supplies should fail.
A portion of a conventional fuel control system is illustrated in FIG. 5 which includes a main fuel pump 200, an auxiliary fuel pump 202, an engine fuel outlet line 204 carrying fuel toward an engine (not shown), a main fuel inlet line 206 carrying fuel from main fuel pump 200 to engine fuel outlet line 204 and an auxiliary fuel inlet line 208 carrying fuel from auxiliary pump 202 to engine fuel outlet line 204. A main fuel check valve 210 is mounted in a main fuel check valve path 212 parallel to main fuel inlet line 206 such that fuel can flow in a first direction from main fuel pump 200 toward engine fuel outlet line 204 but is substantially blocked from flowing in a direction from engine fuel outlet line 204 toward main fuel inlet line 206.
Auxiliary fuel inlet line 208 includes a valve body 214 comprising a selector valve for determining whether main fuel inlet line 206 or auxiliary fuel inlet line 208 provides fuel to the engine that is slidable between a first position, illustrated in FIG. 5, substantially blocking flow through auxiliary fuel inlet line 208 and a second position, shifted in the direction of arrow 216 in FIG. 5, allowing fuel to flow from auxiliary fuel inlet line 208 to engine outlet line 204 through the center portion of the valve body 214. A spring 215 biases valve body 214 toward the first position. As will be appreciated from FIG. 5, valve body 214 is shifted into the position illustrated in FIG. 5 when pressure in main fuel inlet line 206 overcomes the force of spring 215 applied against valve body 214. A pressure limiting valve 218 is located in auxiliary fuel inlet line 208 between valve body 214 and engine fuel outlet line 204 for limiting the pressure of fuel entering engine fuel outlet line 204 from auxiliary fuel inlet line 208.
An auxiliary fuel bypass line 220 extends from a first point 222 in auxiliary fuel inlet line 208 upstream of valve body 214 to a second point 224 in auxiliary fuel inlet line 208 downstream of valve body 214 to bypass valve body 214. An electrohydraulic servovalve 226 includes a second stage spool 228 in auxiliary fuel bypass line 220 and controllably opens and closes the auxiliary fuel bypass line 220.
On engine start-up, main fuel pump 200 does not supply sufficient fuel for engine operation, and therefore auxiliary fuel pump 202 pumps fuel through auxiliary fuel inlet line 208 to valve body 214. A spring shifts the valve body open against the lower pressure of fuel in main fuel inlet line 206 so that fuel can flow to engine fuel inlet 204 and to the engine. The increased pressure in engine fuel outlet line 204 relative to check valve path 212 further biases check valve 210 toward its closed position. After start up, when main fuel pump 200 begins supplying fuel to main fuel inlet line 206 at an adequate rate, the pressure in main fuel inlet line 206 increases and forces check valve 210 open and valve body 214 into the position shown in FIG. 5 substantially blocking fuel flow from auxiliary fuel inlet line 208. Auxiliary pump 202 maintains pressure in auxiliary fuel inlet line 208, and, if main pump 200 fails or pressure in main fuel inlet line 206 falls for other reasons, spring 215 biases valve body 214 in the direction of arrow 216 to open a flow path from auxiliary fuel inlet line 208 to engine fuel outlet line 204 so that the supply of fuel to the engine is not interrupted.
If pressure in the main fuel inlet line drops, as described above, auxiliary fuel will be provided from the auxiliary fuel inlet line 208. However, under some operating conditions it is desirable to minimize delay in this switch from one fuel system to the other and therefore the fuel supply is operated in the configuration illustrated in FIG. 5. In this configuration, valve body 214 is positioned to block fuel the auxiliary fuel flow from auxiliary fuel inlet 208; however, spool 228 is positioned to open auxiliary fuel bypass line 220 so that an auxiliary fuel flow can bypass valve body 214 and flow into engine fuel outlet 204 via pressure limiting valve 218. Because the pressure from main fuel inlet line 206 is greater than the pressure of fuel exiting pressure limiting valve 218, main fuel check valve 210 remains open even when fuel is provided from two sources in this manner. This arrangement helps ensure a continuous fuel supply even if one of the two fuel systems fails.
The above described system functions adequately for its intended purpose. However, when designing fuel systems for gas turbine engines, especially when these engines are used on aircraft, it is generally desirable to reduce the weight of the fuel system. It would therefore be desirable to provide a gas turbine engine fuel supply system that operates in the start up, normal, backup and combined modes described above but that weighs less than a conventional system.